Blender fail to close if a process is forked from it!
Well, I solved it with daemonizing the child process.

class serverProcess(mp.Process):
    def __init__(self, port=9012, timeout=5.0, host=socket.gethostname(), daemon=True):
        mp.Process.__init__(self, daemon=daemon)
        self.port = port
        self.timeout = timeout
        self.host = host

# many other stuff

RPCserverProcess = serverProcess(port=9010, timeout=3.0)